The ageing of such an exhaust gas analyzer probe leads to a longer response time with respect to changes in the exhaust-gas composition, i.e., in the dynamic response of the exhaust gas analyzer probe. However, a rapid reaction of the entire control loop, and thus also of the exhaust gas analyzer probe, to changes in the exhaust-gas composition is a prerequisite for operating the internal combustion engine in a manner that complies with the legal requirements governing the emission of pollutants. The dynamic diagnosis of an exhaust gas analyzer probe thus constitutes a central diagnosis function.
German Patent Application No. DE 197 22 224 describes a method for diagnosing an exhaust gas analyzer probe, which is situated in the exhaust gas of an internal combustion engine behind a catalytic converter and which is sensitive with respect to at least one exhaust-gas component. The rate of change at which the signal of said exhaust gas analyzer probe responds to changes in the concentration of the at least one exhaust-gas component is used as evaluation criterion. The ageing state of the exhaust gas analyzer probe is evaluated via, for instance, the gradient of the probe signal following the beginning of an overrun phase with fuel cutoff.
In another known method, the deviation in a high-frequency excitation of the exhaust-gas analyzer probe for reaching the end amplitude of the measuring signal is evaluated.
Both methods provide stable information only when the control loop of the exhaust gas analyzer probe has no significant effect on the gradient or the amplitude of the measuring signal of the exhaust gas analyzer probe. However, in reality both methods supply a fuzzy result because of the controller reaction and the stochastics due to the evaluation of a single signal. This may not only have the result that an aged exhaust gas analyzer probe is not detected in time but also that a still intact exhaust gas analyzer probe is deemed faulty and is exchanged.
It is an objective of the present invention to provide a method of the type mentioned above, which allows a reliable diagnosis of the dynamic response of an exhaust gas analyzer probe.